Numerous solutions for this purpose are known according to the prior art. For highly precise axial length measurement, solutions which are based on methods of optical coherence tomography (OCT), partial coherence interferometry (PCI) or the like have become established in the prior art.
The basic principle of the OCT method is based on white light interferometry and compares the propagation time of a signal with the aid of an interferometer (generally a Michelson interferometer). The arm with a known optical path length (=reference arm) is used as a reference for the measurement arm. The interference of the signals from both arms produces a pattern from which the relative optical path length within an A-scan (single depth signal) can be read out. In the one-dimensional scanning grid methods the beam is then guided, as in ultrasound technology, transversely in one or two directions, so that a two-dimensional B-scan or a three-dimensional tomogram (C-scan) can be recorded. In this case the amplitude values of the individual A-scans are typically represented in logarithmized grey-scale or pseudocolour values.
If further measurement variables are required in addition to the axial length (AL), central radii of front of the cornea (K), anterior chamber depth (ACD) and limbus diameter (WTW), these further variables may be determined for example from keratometric or topographical image recordings of the eye.
Although these further measurement variables and the OCT measured values are measured by different instruments, the integration of the measurement both of the OCT and also of the further measurement variables in one instrument facilitates simpler handling, for example only one single alignment of the instrument with the patient and an improved lateral registration of the OCT measured values with the further measured values.
However, in a combined instrument the different measurement modalities should not influence one another during the measurement. An influence may be specifically manifested in that light from the OCT measuring system can be seen in one or more images of the other measuring system for example as a bright spot, which can disrupt the measurement. In order to ensure this absence of influence 2 groups of solutions are possible:
In a first group of solutions the different recordings are made sequentially, i.e. one after the other.
One example is shown in US 2005/0203422 A1, which shows a combined system comprising keratometer and OCT tomography. In order to separate the two modalities from one another, a chronological separation is likewise proposed here.
A further example is the IOLMaster from Carl Zeiss. This is a combined instrument which determines the keratometry, the axial length by means of PCI (partial coherence interferometry) and the anterior chamber depth by means of slit-lamp illumination and image detection, as well as further parameters of the eye such as the so-called white-to-white-distance.
With all these measurements which take place sequentially the time spent on the measurements is longer. Moreover, it is disadvantageous that the different measurements of OCT and ultrasound and/or keratometry could take place on the basis of possible eye movements at slightly different places. In general, therefore, a reproducibility of the measurement is accordingly difficult to achieve.
In a second group of solutions, the different recordings are made simultaneously, for which the measurement systems must have a corresponding optical separation.
As a further example, in US 2005/0018137 A1 a combined system comprising keratometer and axial length measurement by means of PCI is described. In this case the separation of both modalities is achieved by beam splitting by means of polarization separation.
The above-mentioned US 2005/0203422 A1 also mentions, as an alternative to the sequential measurement of the modalities (by means of OCT and keratometry), a separation of the modalities by a dichroitic beam splitter.
In all these examples an optical separation of the different measurement systems takes place either by the use of different wavelengths or by means of additional optical elements which prevent the measurement systems from influencing one another.
This is disadvantageous in such systems in that correspondingly higher demands are made on the optics and/or camera, which can have a negative effect on the producibility and/or price level thereof.